Microorganisms belonging to the class Actinobacteria, commonly called actinomycetes, reside taxonomically within the Gram-positive bacteria and are ubiquitous in terrestrial environments. Actinomycetes are a prolific source of diverse biologically active metabolites. They have been a source of a numerous useful products including pharmaceuticals, agrichemicals, low molecular weight enzyme inhibitors, immunomodifiers, and enzymes for use in a number of industrial applications, from the food industry to paper making. These microorganisms have also been useful in agriculture as a means of pathogen protection and growth enhancement. Although many useful substances have been discovered from soil actinomycetes over the last 60 years, the yield of novel products has drastically decreased as common soil species continually yield previously discovered metabolites. For this reason, there has been a major effort to discover new actinomycete taxa in the hope that these microorganisms will provide a new source of useful products (Bull et al., 2000).
Actinobacteria are one of a number of classes of bacteria. The class Actinobacteria can be further subdivided into six orders, including the Actinomycetales which can be broken down into 10 suborders. Classical methods for determining taxonomic novelty include morphological and physiological criteria such as color, presence or absence of mycelia, hyphal branch characteristics, spore pattern and motility, tolerance of variation in temperature, salinity and pH, and the ability to utilize various substrates. Although these criteria remain an important component of taxonomic analyses, a new and more definitive method to establish strain uniqueness is 16S rDNA sequence analysis, which also provides evolutionary information on the isolate (Stackebrandt, et al., 1997).
Membership of a strain within the class Actinobacteria is indicated by 16S rDNA sequence similarity values above 80%, as determined by comparison of almost complete 16S rDNA sequences with the most deeply branching members of the class, and the presence of signature nucleotides (Stackebrandt, 1997). Signature nucleotides specific for a taxonomic group are chosen for their presence in more than 95% of the members of that group. 16S signature nucleotide sequences can be used on various taxonomic levels, from defining an order of bacteria to the subdivision of families into genera. This method provides a powerful mathematical model of bacterial evolution and an objective, rather than subjective, set of rules by which bacteria may be assigned a taxonomic status within the classification system.
Despite the fact that the oceans cover 70% of the earth's surface, all known actinomycete genera discovered to date have been land dwellers. In fact, only one marine actinomycete species has been described (Helmke and Weyland, 1984) and it belongs to a well-known terrestrial genus. Although actinomycetes have been cultured from marine sediments, it is widely believed that marine isolates are derived from dormant terrestrial spores that were washed into the sea (Goodfellow and Haynes, 1984). The “wash-in” theory was postulated because the marine isolates did not require seawater for growth, were closely related to terrestrial species, and tended to decrease in number with increasing distance from land (Goodfellow and Williams, 1983). Because many terrestrial actinomycetes can tolerate high salinity and pressure, and because of their distribution and physiology, it was concluded that most actinomycetes have been washed into the sea and collect in sediments where they can survive for long periods of time as spores (Goodfellow and Haynes, 1984). These types of studies have led to the general belief that marine actinomycetes are not significantly different from those on land and therefore of little utility as a source of novel industrial products.